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This paper deals with the measurement of N 2 (A 3 + u ) metastable state density in a dielectric barrier discharge in nitrogen and nitrogen with small admixtures of oxygen, operating in a Townsend-like discharge regime. The measurement is made by optical-optical double resonance-LIF, calibrated by a method based on the measurement of the ratio of nitrogen second positive system and NO-γ emissions, and of NO density by LIF. A metastable density of the order of 10 13 cm −3 was found in a nitrogen diffuse discharge. Addition of small oxygen concentrations to the discharge drives a transition to the filamentary regime that appears to be caused not by a marked decrease of the metastable density in the discharge but rather by a considerable increase of its quenching rate. Such an increase, due to collision quenching by O 2 and O, strongly reduces the survival of the metastable between two discharge pulses. These observations are consistent with the idea that the diffuse regime can be due to a space charge memory effect due to the nitrogen triplet metastable, which is cancelled by the introduction of oxygen in the gas feed.

Rate constants for N2(A,v) quenching by O, for levels v=2–7, by O2 for levels v=3–7, and by NO for levels v=2–4, have been measured in this work. This is the first data set for the quenching by O of vibrational levels v>3. The results of this work are based on the measurement by laser induced fluorescence (LIF) of N2(A,v) decay in a rf pulsed postdischarge, supported by LIF measurements of NO density. O atom density is deduced by N2(A,v=0,1) decay using the known rate constants of N2(A,v=0,1) quenching by O, O2, and NO. Finally, from appropriate scaling of LIF results for the various v levels, N2(A,v) vibrational distributions are deduced, showing a quite low vibrational excitation of the triplet metastable, characterized by an average Boltzmann vibrational temperature of the order of 2000–2500 K with some superimposed structures.

A possible way to store both renewable energy and CO 2 in chemical energy is to produce value-added chemicals and fuels starting from CO 2 and green electricity. This can be done by exploiting the non-equilibrium properties of gaseous electrical discharges. Discharges, in addition, can be switched on and off quickly, thus being suitable to be coupled with an intermittent energy source. In this study, we have used a nanosecond pulsed discharge to dissociate CO 2 and CH 4 in a 1:1 mixture at atmospheric pressure, and compared our results with literature data obtained by other discharges. The main products are CO, H 2 , C 2 H 2 , water and solid carbon. We estimate an energy efficiency of 40% for syngas (CO and H 2 ) production, higher if also other products are considered. Such values are among the highest compared to other discharges, and, although not very high on an absolute scale, are likely improvable along possible routes discussed in the paper and by coupling to the discharge a heterogeneous catalysis stage.

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